Double Bass Reversing Phase for 1 of 2 Inputs

[Steve Freides]

I'd appreciate a primer on Phase. I was using my AI Ten2 yesterday, not for my bass, but the question really isn't bass-specific. I was using both channels as you might, e.g., if you had a pickup setup with a piezo element on each side of your bridge and plugged one of those into each of the two channels of your two-channel amplifier. Or maybe a pickup and a mic. In my case, it was the stereo output, split, of a church organ's headphone amplifier.

When I reversed the phase on either channel, the hum disappeared. I some have experience in the guitar world with this - if pickups aren't humbuckers, and you use two of them, the hum disappears, e.g., pickup selector positions 2 and 4 on a Strat or similar.

My question: Other than losing the _hum_, what _else_ do you lose in your signal when you reverse phase in one of two inputs? Anything and, if so, what? Is phase-reversing one of two channels the best solution or would some sort of notch filter in combination with both channels being left in phase be better?

Thanks in advance.

-S-

 

[mattwells]

Here is a thread on it: Help Me Understand Phase Reversal

My only experience has been using phase reversal to tame feedback.

 

[Steve Freides]

@mattwells, that was interesting reading, but it doesn't completely address my specific question. In my case, I was asking about phase reversing one of the two sides of a stereo feed. The closest thing in that other thread was about a mic and a DI sound and nudging waveforms to make them line up. The reason I asked is that I assumed reversing the phase of half a stereo feed would have a pretty disastrous effect on the sound, but it didn't, so far as I could tell.

-S-

 

[FLORINS]

If you combine two signals that are out of phase you will get frequency cancellations. In the case of a stereo recording if you were to reverse the phase on just one of the channels (L or R but not both) any part of the signal that is the same in both channels would be nulled. For example, if you were to take a typical recorded pop song and split it into mono L and R channels and flip the phase on one of these channels the instruments that are panned up the middle like the kick, snare, bass and lead vocal would be cancelled out. You would be left with material that was panned L and R as well as any stereo reverb or ambience in the track. If you have a basic DAW (Pro Tools etc) you can do this experiment.
It's helpful also to make a distinction between electrical polarity and phase. Please see article Invalid Link Removed.

 

[megafiddle]

...
The reason I asked is that I assumed reversing the phase of half a stereo feed would have a pretty disastrous effect on the sound, but it didn't, so far as I could tell.

-S-

If the signals in the stereo channels are uncorrelated, then reverving the phase of one will make little or no audible difference. For example, a different instument in each channel would be uncorrelated.

In a music recording, a phase reversal can have a serious effect, because there is often certain instruments recorded in mono, like bass and lead vocals. When these signals are mixed out of phase you can get a cancellation.

It sounds like the left and right signals from the organ are somehow different enough, so that the signal in one channel has nothing in the other channel to be out of phase with.

-

 

[mattwells]

@mattwells, that was interesting reading, but it doesn't completely address my specific question. In my case, I was asking about phase reversing one of the two sides of a stereo feed. The closest thing in that other thread was about a mic and a DI sound and nudging waveforms to make them line up. The reason I asked is that I assumed reversing the phase of half a stereo feed would have a pretty disastrous effect on the sound, but it didn't, so far as I could tell.

I think i'm confused on your chain here. Organ => Stereo out (split coming out of the organ?) => AI Ten2 (one into each channel)? Does the AI Ten2 you have allow you to run both channels at once into the master (quick google turned up a few different versions of amps that were in that combo)?

If the signal is being split, one side run out of phase, and then recombined - I believe you get near-silence. So something else is going on.

 

[Steve Freides]

It sounds like the left and right signals from the organ are somehow different enough, so that the signal in one channel has nothing in the other channel to be out of phase with.

I think that's what was happening.

If the signal is being split, one side run out of phase, and then recombined - I believe you get near-silence. So something else is going on.

Apparently I got less bass but the pan of each stop in the signal seems to have been sufficient to still produce quite a bit of sound.

I think i'm confused on your chain here. Organ => Stereo out (split coming out of the organ?) => AI Ten2 (one into each channel)? Does the AI Ten2 you have allow you to run both channels at once into the master (quick google turned up a few different versions of amps that were in that combo)?

Yes - it's a two-channel amp, and I was using a cable with a stereo 1/4" plug at one end (tip/ring/sleeve), and two male 1/4" plugs at the other end, one each labeled Tip and Ring. Pushing the phase button on either channel's most immediate effect was to kill the hum while still leaving much of the music.

For those unfamiliar with electronic church organs, because they're attempting to mimic the spatial arrangement of the pipes of a pipe organ, there's a fair amount of panning of the signal going on, e.g. it's typical that a certain stop will come mostly out of one side or the other side of a pair of stereo speakers.

-S-